wii backup manager link in my signature. HERE is a link to the guide for it too. Make sure to pay attention when you click the link, they toss a bunch of crap out there to try to get you to click the wrong thing.

That is because your HDD is formatted to WBFS. Wii backup manager will read it though. On Wii backup manager click on Drive 1, Then click on the drop down box that says "inactive" on it. A list will drop down choose your HDD Drive letter from there, and wii backup manager will load your HDD.